Add first pass at actions to handle form POST requests.
Update form POST handler to bail out if AJAX, and use Achievements-specific action name.
Tweak to phpdoc wording
First pass at redeem achievement feature. See #62

* Adds a widget and shortcode.
* Adds new template (form-redeem-code.php).
* Achievement post type metabox still needs updating to allow entry of
the redemption code.